1. Field of the Invention
The invention is related to the field of flow meters, and in particular, to Coriolis flow meters.
2. Description of the Prior Art
Coriolis flow meters determine mass flow rates by sensing the Coriolis forces on a vibrating conduit. The conduit consists of one or more tubes and is forced to vibrate at a resonant frequency. The resonant frequency of the tube(s) is proportional to the density of the fluid in the flow tube(s). Sensors located on the inlet and outlet sections of the tube(s) measure the relative vibration between the ends of the tube(s). During flow, the vibrating tube(s) and the flowing mass couple together due to the Coriolis forces, causing a phase shift in the vibration between the ends of the tube(s). The phase shift is directly proportional to the mass flow.
There is a secondary pressure effect on the flow tube(s) of a Coriolis sensor. A change of pressure, without a change in mass flow rate, will change the effect of the bending forces on the tube. When the pressure increases, the flow tube(s) will stiffen and the same amount of Coriolis forces due to the constant mass flow rate will cause a smaller amount of bending in the tube(s). When the pressure decreases, the flow tube(s) will become more flexible and the same amount of Coriolis forces due to the constant mass flow rate will cause a larger amount of bending in the tube(s). The flow pressure effect is linear and is typically stated as a percent of the flow rate per unit change of pressure. Correcting for the pressure effect requires either using an average pressure value or measuring the actual pressure. Using an average pressure value can induce unacceptable errors when there are large variations in the operating pressure in the system. Measuring the actual pressure typically requires a pressure sensing port, a pressure transducer, electronic circuitry designed to monitor the pressure transducer, and some means for transferring the measured pressure to the Coriolis flow meter.
Therefore there is a need for a better system and method for determining the pressure in a Coriolis flow meter.